David Miranda, who was recently detained while carrying British intelligence documents through London's Heathrow Airport, reportedly wrote down the password to one of the encrypted files on a piece of paper seized by police.

Miranda, partner of The Guardian reporter Glenn Greenwald, carried a "piece of paper containing basic instructions for accessing some data, together with a piece of paper that included the password for decrypting one of the encrypted files on the external hard drive," UK Deputy National Security Adviser Oliver Robbins said in a "statement prepared for a High Court hearing," according to the BBC.

Robbins said one file Miranda was carrying included 58,000 "highly classified UK intelligence documents," but it's not clear how many documents were part of the file said to be associated with the password.

For his part, Greenwald denied that the password on its own could decrypt a document. "Anyone claiming that David Miranda was carrying a password that allowed access to documents is lying. UK itself says they can't access them," Greenwald wrote on Twitter. In another tweet, he wrote, "Good encryption requires multiple passwords, not just one. That pw allows no access to documents, period."

Nonetheless, Robbins chided Miranda, saying that carrying the piece of paper was "a sign of very poor information security practice," according to The Telegraph.

Miranda had been traveling to Berlin to visit documentary filmmaker Laura Poitras. Miranda "was in Berlin to deliver documents related to Mr. Greenwald’s investigation into government surveillance to Ms. Poitras, Mr. Greenwald said," the New York Times recently reported. "Ms. Poitras, in turn, gave Mr. Miranda different documents to pass to Mr. Greenwald."

The UK seized Miranda's computer, hard drive, and other pieces of equipment containing the documents. Robbins said that the material seized was "highly likely to describe techniques crucial in life-saving counter-terrorist operations," the BBC noted. "He added that intelligence staff were identifiable in the information, posing a danger to them in the UK and overseas."

Miranda will argue at a court hearing in October that the government misused its powers. In the meantime, an agreement approved by Miranda and his lawyers allows police to keep examining the seized material for national security purposes until the hearing.